specifity of iodinated antibodies

I routinely perform western blots with a commercially available monoclonal
anti-phosphotyrosine antibody for the detection of tyrosine phosphorylation of
subunits of the nicotinic acetylcholine receptor. Recently I observed that the
iodinated form of the same primary antibody I used before with other detection
systems showed a preferential binding for one tyrosine-phosphorylated subunit
over the other tyrosine-phosphorylated subunits. Cross-reactions between the
formerly used secondary antibodies and the receptor subunits were of course
checked and can be excluded. Did anybody else observe such change of antibody
specifity due to iodination? Could anybody explain?
Mathias Dreger